With the increasing down-scaling of integrated circuits and the increasingly demanding requirements to the speed of integrated circuits, transistors need to have higher drive currents with increasingly smaller dimensions. Fin Field-Effect Transistors (FinFET) were thus developed. The FinFETs include vertical semiconductor fins above a substrate. The semiconductor fins are used to form source and drain regions, and channel regions between the source and drain regions. Shallow Trench Isolation (STI) regions are formed to define the semiconductor fins. The FinFETs also include gate stacks, which are formed on the sidewalls and the top surfaces of the semiconductor fins.
Since the aspect ratio of the gaps between the fins becomes increasingly greater, in the gap filling for forming the STI regions, materials with high shrinkage rates are often used. The high-shrinkage-rate materials shrink significantly when annealed. This causes significant stresses applied on the fins, and hence the fins may have deformation and cracking.